In recent optical transmission systems, the capacity of transmission is increased by multilevel modulation. Multilevel modulation is a technique which increases the capacity of transmission depending on the number of modulation levels by modulating the amplitude and/or phase of an optical signal into multiple levels. However, as the number of modulation levels increases, the receiver sensitivity worsens, resulting in a shorter transmission distance.
Therefore, next-generation optical transmission systems are expected to increase the capacity of transmission by using not only a multilevel modulation technique but also a polarization multiplexing technique. Polarization multiplexing is a technique which combines optical signals with different polarized waves (planes in which light waves vibrate) to increase the capacity of transmission. Usually, two optical signals whose polarized waves are orthogonal to each other are combined to double the capacity of transmission. A polarization multiplexing technique that uses two optical signals with mutually orthogonal polarized waves is called orthogonal polarization multiplexing. As described above, the use of polarization techniques is attractive for next-generation large-capacity optical transmission systems.
On the other hand, it is known that polarization causes various problems related to signal degradation. For example, polarization dependent loss (PDL) in optical waveguides, polarization dependent gain (PDG) in optical amplifiers, and polarization hole burning (PHB) give loss or gain to an optical signal depending on its polarization. Furthermore, polarization mode dispersion (PMD) in optical fibers or optical waveguides causes a delay in an optical signal depending on its polarization.
The polarization dependence of an optical fiber is attributable to the fact that stress on the optical fiber deforms its core cross section. Such cross-sectional deformation of the core not only causes polarization mode dispersion but also changes the polarized wave of the optical signal depending on its wavelength and polarization. Also, it is known that since generally the stress on the optical fiber is changing, the polarization dependence of the optical fiber also changes with time.
These phenomena caused by polarization can seriously deteriorate polarization multiplexed signal generated by polarization multiplexing. For example, PDL causes different losses in multiplexed signals for polarization multiplexed signal and also changes the polarization state between multiplexed signals. This phenomenon is explained below referring to FIGS. 1A to 1D. Let's suppose that a PDL device 001 transmits 100% of optical power of a horizontally vibrating TE polarization and 33.3% of a vertically vibrating TM polarization. When polarization multiplexed signal as shown in FIG. 1A which combines an optical signal with a TE polarization and one with a TM polarization enters the PDL device 001, there is a difference in light intensity between the two multiplexed polarized waves as shown in FIG. 1B. In this case, the polarization state between multiplexed signals is maintained. On the other hand, when polarization multiplexed signal as shown in FIG. 1C which combines an optical signal with a 45°-rotated X polarization and one with a 45°-rotated Y polarization enters the PDL device 001, the polarized waves of the two multiplexed signals rotate in opposite directions and the angle between the polarized waves changes from 90 degrees to 120 degrees as shown in FIG. 1D. This phenomenon can be interpreted to suggest that the X and Y polarizations as shown in FIG. 1C are both a combination of TE and TM polarizations and their TM polarization components are reduced by the PDL device 001 and consequently they both become closer to the TE polarization. In fact, when the PDL device 001 does not transmit the TM polarization, X and Y polarizations both become the TE polarization.
Polarization of an optical signal can be visualized by a Poincare sphere as shown in FIG. 2A. The Poincare sphere is a visualization tool which uniquely represents a polarized wave 009 as a point on a spherical surface. For example, FIG. 2A shows TE polarization 00A, TM polarization 00B, +45° polarized wave 00C, −45° polarized wave 00D, right-handed circular polarized wave 00E, and left-handed circular polarized wave 00F on the Poincare sphere. The line connecting the TE polarization 00A and TM polarization 00B, the line connecting the +45° polarized wave 00C, and −45° polarized wave 00D, and the line connecting the right-handed circular polarized wave 00E and left-handed circular polarized wave 00F are called S1, S2, and S3 axes respectively, in which these axes cross perpendicularly at the center of the Poincare sphere. Polarized waves orthogonal to each other like the TE polarization 00A and TM polarization 00B are expressed by points located on opposite sides. Polarization dependence can be understood to be a property that loss or delay varies depending on the position of a point on the Poincare sphere.
Polarization scrambling is known as a technique to suppress signal degradation caused by polarization dependence. Polarization scrambling is a technique to change the polarization of an optical signal in order to prevent the polarized wave of the signal from being fixed in a certain state. Therefore, the polarized wave of a polarization-scrambled optical signal has a temporal distribution 00H-1 as shown in FIG. 2B. Ideally, the polarization of the optical signal should be changed so that the optical signal polarized waves appear uniformly in a distribution 00H-2 covering the whole Poincare sphere as shown in FIG. 2C. Consequently the polarization dependence of the optical signal is averaged, thereby suppressing signal degradation caused by polarization dependence. Also, as for mutually orthogonal polarized waves, their polarization dependences are generally reverse, so it is effective to use a polarization scrambler which rotates the waves cyclically on a circumference with an axis passing through the center of the Poincare sphere as the axis of rotation (for example, distribution 00H-3 shown in FIG. 2D).
FIG. 3 shows a typical form of an optical transmission system which uses a polarization scrambling technique. In this system, an optical modulator (Mod) 003 modulates continuous light coming from a laser light source (LD) 002 according to transmission data and outputs it as an optical transmission signal. The polarized wave of the optical transmission signal is constant. The optical transmission signal enters a polarization scrambler (PS) 004 in which the polarized wave of the signal is temporally rotated. This polarized wave rotation process is called polarization scrambling. The optical transmission signal polarization-scrambled by the polarization scrambler 004 enters an optical fiber transmission path 005. Several optical repeaters (Nodes) 006 are inserted midway in the optical fiber transmission path 005 so as to compensate for signal degradation caused by loss or wavelength dispersion in the optical fiber transmission path 005. In some cases, an optical repeater 006 has a polarization scrambler 004. Then, the optical transmission signal which has entered the optical fiber transmission path 005 passes through the path 005 before it is received by an optical receiver (Rx) 008. The optical receiver 008 demodulates the transmission data in the received optical signal. If the polarization of the optical signal which the optical receiver 008 receives is to be limited, a polarization tracer (Pol. Tracer) 007 is inserted just before the optical receiver 008 to eliminate a fluctuation in the polarization of the optical signal so as to make the polarized wave of the optical signal suitable for the optical receiver 008 in advance. Some types of optical receiver 008 do not require an external polarization tracer 007 since they incorporate a polarization tracing function.
The optical fiber transmission path 005 and optical repeater 006 have different types of polarization dependence because they include an optical fiber, optical amplifier or optical waveguide type device. Since the influence of such polarization dependence depends on the polarization of the optical signal, if the polarization of the optical signal is unchanged, a state in which signal degradation due to polarization dependence is maximized may continue. Since polarization scrambling of the optical transmission signal by the polarization scrambler 004 changes the polarization of the optical signal, the influence of polarization dependence of the optical fiber transmission path 005 can be averaged. This effect can be enhanced when an error correction technique such as forward error correction (FEC) is combined with polarization scrambling. For example, FEC is a technique which divides the optical transmission signal into FEC frames of several microseconds before transmission and corrects an error of the received signal on a frame-by-frame basis. If the optical signal is polarization-scrambled at a much higher speed (for example, 10 MHz or more) than the FEC frame length, an error in the received signal at the moment when the polarization of the optical signal becomes the worst can be corrected using a received signal at another time for the optical fiber transmission path 005 or optical repeater 006. Also, since response to change in the polarization of the optical signal is slow, at least PDG or PHB can be suppressed by polarization scrambling at a much higher speed (for example, 100 kHz or more) than the response speed.
Thus, polarization scrambling is an effective technique for suppression of signal degradation caused by polarization dependence. As described above, it is desirable to perform polarization scrambling at high speed e. It is reported that signal degradation is more effectively suppressed when polarization scrambling is synchronized with data modulation and the polarization scrambling speed is made equal to the data modulation speed, as described in Japanese Patent No. 3375811.
However, an ordinary polarization scrambler mechanically drives its internal devices, so its polarization rotation speed is in the range from several kilohertz to several megahertz. Not many polarization scramblers are able to rotate polarized waves at higher speed with accuracy, though electro-optical polarization scramblers are known to be able to rotate polarized waves at a speed of 10 MHz or more.
A typical electro-optical polarization scrambler is a polarization modulator which modulates the phase difference between two mutually orthogonal TE and TM polarizations using an optical phase modulator. This polarization modulator can rotate polarized waves on a circumference 00H-4 (FIG. 4A) with the S1 axis as the axis of rotation on the Poincare sphere. The circumference 00H-4 includes a circumference 00G-3 which passes through +45° polarized wave 00C, and right-handed circular polarized wave 00E, in which the +45° polarized wave 00C, and right-handed circular polarized wave 00E can be converted into each other. However, TE polarization 00A and TM polarization 00B cannot be converted at all. Another problem is that even the +45° polarized wave 00C, and right-handed circular polarized wave 00E cannot be converted, for example, into TE polarization 00A.
In this connection, a non-patent document authored by E. Hu et al. and entitled “4-Level Direct-Detection Polarization Shift-Keying (DD-PolSK) System with Phase Modulators” (OFC, FD2, 2003) reports a polarization modulator which modulates the amplitude ratio and phase difference between mutually orthogonal TE polarization 00A and TM polarization 00B. This polarization modulator simulates polarization rotation on a circumference 00H-5 (FIG. 4B) with the S3 axis as the axis of rotation on the Poincare sphere by varying the amplitude ratio between the TE polarization 00A component and TM polarization 00B component of an incoming optical signal. Here, it is also possible to output an optical signal with a desired polarized wave by modulating the phase difference between the TE polarization 00A component and TM polarization 00B component in addition to polarization rotation with the S1 axis as the axis of rotation. However, an optical signal entering the polarization modulator must have a TE polarization 00A component and a TM polarization 00B component. The polarization of an incoming signal with TE polarization 00A component and TM polarization 00B component cannot be converted arbitrarily. Desirably the incoming polarized wave should be a +45° polarized wave 00C, equally having the TE polarization 00A component and TM polarization 00B component.